1. Field of the Invention
The present invention relates to an encoder which detects an angle of rotation or the like and which also detects occurrence of an electrical malfunction.
2. Description of the Related Art
An encoder has been used, for example, in a steering angle sensor which detects a steering angle of a vehicle. Such an encoder cannot be used in a control that requires reliability unless it gives information as to whether malfunction such as break or short circuit exists therein as well as simply detecting the angle, speed, or direction of rotation.
Therefore, a conventional encoder is provided with a malfunction-detecting circuit in its inside where a malfunction signal detected by the encoder is outputted to an electronic control unit by a signal line separate from a signal line of encoder outputs with regard to the steering wheel sensor.
However, with the above encoder there is a problem that dedicated signal line is needed, and in order to solve the problem, the following technology is proposed in Japanese Unexamined Patent Publication No. 2002-357456,for example. In an encoder which generates each pulse signal for phase A and phase B, which are displaced with each other by a predetermined angle, and for phase Z at a predetermined angle, a malfunction detection signal is superimposed on the phase Z to identify the occurrence of malfunction from an output voltage of the phase Z.
This encoder is provided with two comparators for the phase A and the phase B, and includes an OR circuit which obtains a logical sum of the outputs and one transistor which is driven by the OR circuit to add the transistor output to phase Z. Consequently, the dedicated signal line which sends the malfunction detection signal to the electronic control unit is unnecessary.
However, if the above-mentioned transistor breaks down, any malfunction in both the phase A and the phase B cannot be transmitted, which presents a problem that it is difficult to ensure reliability.